Food-grade tertiarybutyl hydroquinone ("TBHQ") is useful as a flavor and aroma preservative. Food-grade TBHQ is required by the Food Chemicals Codex Specifications to have at least 99.0% (wt) monotertiarybutyl hydroquinone ("MTBHQ") on a dry basis and less than 0.2% (wt.) of the major impurity, 2,5-ditertiarybutyl hydroquinone ("DTBHQ"). Additional Food Chemicals Codex Specifications for TBHQ are as follows: 0.2% (wt.) maximum t-butyl-p-benzoquinone, 10 ppm maximum heavy metals, melting range between 126.5.degree. C. and 128.5.degree. C., 0.1% (wt.) maximum hydroquinone, 0.0025% (wt.) maximum toluene, and an ultraviolet absorbance for polynuclear hydrocarbons with maximum absorbances at four different wavelength regions. As a result of such low impurity requirements, the production of relatively pure MTBHQ is essential to the production of food-grade TBHQ.
MTBHQ production is well known in the art. For example, U.S. Pat. No. 2,722,556 to Young, et al., the disclosure of which is incorporated herein by reference in its entirety, discloses a process for producing MTBHQ by reacting hydroquinone with isobutylene or tertiarybutyl alcohol. The crude MTBHQ produced by this process does not meet the TBHQ Food Chemicals Codex Specifications because of high impurity levels. The main impurity in the crude MTBHQ, DTBHQ, is produced in large quantities during the butylation of hydroquinone reaction.
One method of purifying MTBHQ is discussed in the background section of U.S. Pat. No. 5,087,771 to Hilderbrand. This reference discloses purification of crude MTBHQ by dissolving crude, water-wet MTBHQ crystals in toluene, decanting the water, and filtering the hot solution to a crystallizer. The majority of the DTBHQ remains in the toluene mother liquor as the MTBHQ crystallizes. This process is expensive and is very intensive in both labor and equipment, requiring long batch cycle times and specialized equipment.
An alternative method of purifying MTBHQ is taught in U.S. Pat. No. 5,087,771 to Hilderbrand. In this method, crude, water-wet MTBHQ crystals are contacted with a non-polar solvent, such as naphtha, at an elevated temperature. The MTBHQ crystals form a slurry mixture with the non-polar solvent. The temperature of the mixture is lowered and TBHQ is obtained as a solid from the solvent at a temperature above the crystallization point of DTBHQ. The preliminary isolation step required in this process to obtain the crude MTBHQ crystals results in relatively high processing costs, added processing time and extra equipment usage.
JP 62081339 to Hasegawa et al. also describes the purification of crude MTBHQ. In this process, crude, water-wet MTBHQ crystals are purified by forming an extractable eutectic mixture and subsequent solid-liquid extraction. The eutectic mixture is made possible by the unique properties of MTBHQ and DTBHQ: the melting point of a mixture of MTBHQ and DTBHQ is lower than either the melting point of MTBHQ (126.degree. C.) or DTBHQ (216.degree. C.). Although effective, this method requires careful control of temperature, time and flow rate in addition to the high processing costs due to the required preliminary isolation of crude MTBHQ crystals.
In spite of the foregoing, the need still exists for a cost-effective process to produce purified MTBHQ, particularly for removing DTBHQ, such as an in-process purification of MTBHQ.